High strength and high ductility steel sheet having ultrafine crystalline grain structure obtained by low strain processing and annealing of ordinary low carbon steel, and a method for producing thereof

ABSTRACT

A high strength and high ductility low carbon steel having a tensile strength of 800 MPa or more, an uniform elongation of 5% or more, and an elongation to failure of 20% or more which is produced by a method comprising subjecting an ordinary low carbon steel or an ordinary low carbon steel added with boron in an amount being 0.01% or less and effective for accelerating martensitic transformation to processing and heat treatment to prepare a product having coarser size of austenite crystal grains and then to water-quenching, to provide a steel product having a martensite phase in an amount of 90% or more, and subjecting the steel product to a low strain processing, specifically a cold rolling at a total rolling reduction in thickness of 20% or more and less than 80%, and to a low temperature annealing at 500° C. to 600° C., and a method for producing said high strength and high ductility low carbon steel.

FIELD OF THE INVENTION

The present-invention relates to a high strength and high ductility lowcarbon steel having a tensile strength of 800 MPa or more, an uniformelongation of 5% or more, and an elongation to failure of 20% or morewhich is produced by a method comprising (1) subjecting an ordinary lowcarbon steel or an ordinary low carbon steel added with boron in anamount of 0.01% or less being effective for accelerating martensitictransformation to processing and heat treatment to prepare a steel sheethaving coarser austenite crystal grains and then to water-quenching, toprovide a steel sheet having a martensite phase in an amount of 90% ormore, and (2) subjecting said steel sheet to a low strain cold-rollingof a total rolling reduction of thickness 20% or more and less than 80%,and to a low temperature annealing at 500° C. to 600° C., and a methodfor producing said high strength and high ductility low carbon steel.

In the present invention, the ordinary low carbon steel means a steelwhose carbon content is 0.2% or less, manganese content is 1.6% or less,silicon content is 0.5% or less, phosphorus content is 0.05% or less andsulfur content is 0.05% or less. The ordinary low carbon steel addedwith minute amount (0.01% or less) of boron means the steel produced byadding effective amount of boron necessary for acceleration ofmartensitic transformation in an amount of 0.01% or less to abovementioned ordinary low carbon steel for the purpose to improve thequenching property.

In the present invention, content % means weight %.

BACKGROUND OF THE INVENTION

In recent years, the improvement in usability of vacant spaceaccompanying the high-rise building, energy saving requirement for carsor ships and recycling of natural resources are becoming more requisite,and these tendency is also applicable to the steel materials. To satisfyformer two requirements, it is necessary to make the strength andductility of the steel sheet much higher, and in order to improve inrecycling of natural resources as well, it is necessary to achieve saidimprovement in making the strength and ductility of the steel sheet muchhigher by using ordinary low carbon steel not by adding other alloyingelements.

In order to develop the steel sheet of high degree properties requiredseveral project teams are established. These project teams are named as,for example, Super Metal Project or Super Steel Project and are aimingto develop a ferrite structure steel having “800 MPa” tensile strength,which is two times to ordinary low carbon steel, having high ductility,and having property for easy welding as well, by producing ultra finecrystal grains of 1 μm or less in the present “400 MPa class compositionsteel sheet”.

In the concerned technical field, for the improvement in strength byrefining the ferrite crystal grains of steel, it is well known that therelationship of Hall-Petch equation is realized, that is, yield stressand tensile strength are improved by refining the size of ferritecrystalline of steel and simultaneously the toughness is also improved.However, there is a problem of the elongation falling down in tensiontest.

In CAMP-ISIJ Vol.11 (1998), pp 1031-1034, the following disclosure isreported. In studing to obtain a steel whose strength is improved to 800MPa grade of 400 MPa grade steel with good weldability as a startingmaterial, they settled their object of their study as to accomplish thegrain size of 1 μm or less in ferrite-carbide structure. And, as theconcrete measures to accomplish said object, following process ismentioned. The austenite transforming treatment is carried out on aspecimen having 8 mm thickness, namely, after said specimen is heattreated at the temperature of 1000° C. for 60 sec, cooled down by waterso as to obtain martensite structure, then the biaxial hot rolling iscarried out on the specimen at a total rolling reduction of thickness90% at 640° C. And they reported that the ferrite structure of theobtained steel is characterized to have an equiaxed fine structure, thenominal grain size becomes 0.77 μm and Vickers hardness is 245, which iscorresponding to tensile strength of 760 MPa. However, in saidreference, there is no description reporting the actual measuringprocedure about the tensile strength by preparing a test piece forstrength test from the obtained bulk steel, further, there is no mentionconcerning elongation. Still more, the steel used in said reference isthe steel whose manganese content is increased to 2.03% for the purposeto obtain the quenching ability, further the rolling of the martensitestructure is carried out by hot condition at 640° C.

Further, in the development of a steel which satisfies the requirement,such as high strength, high toughness and high ductility, thesolid-solution hardening method which adds alloy element, theprecipitation hardening method and the transformation strengtheningmethod are being investigated, however, these methods have a problem ofhigh price because of containing high amount of alloy element, furtherhave a problem to deteriorate the recycling property. On the other hand,to solve said problems, the strengthening methods by refining ofcrystalline grains, which are the methods by adding no alloy element,are investigated and reported, however, since these methods are based ona large strain processing, the problem of requiring a particularprocessing equipment arises.

The inventors of the present invention have already investigated aboutthe structure and the mechanical properties of a steel sheet obtained bythe combination of Accumulative Roll-Bonding (called as ARB) at roomtemperature and annealing, which is a large strain processing, using thesteel sheet whose structure is ferrite-pearlite as a starting material.However, since the structure obtained after large strain processing hasa heterogeneous structure in which both a region containing cementiteand a region not containing cementite exist, a heterogeneous mixedgrains structure whose grain size of ferrite are not uniform isgenerated in annealing process, therefore, the expected high strengthand high ductility steel sheet could not be obtained.

The idea of producing the ultra fine ferrite crystalline grainsstructure of ordinary low carbon steel from a martensite structure isnot a novel one, because said idea is also used by STX-21 Project orSuper Metal Project which promotes the development of super steel.However, by said method, the development to accomplish the high strengthand high ductility low carbon steel having a tensile strength of 800 MPaor more, an uniform elongation of 5% or more, and an elongation tofailure of 20% or more has not realized yet. In particular, the idea toobtain a steel having high strength, high ductility and high toughnessis not existing in the concept of these Projects.

The object of the present invention is to provide the steel sheet havingsaid desired properties and a method to produce a steel sheet havingsaid desired properties without big change of the producing plants for aconventional steel sheet.

As mentioned above, the idea to use a steel sheet with martensitestructure as a starting material to realize the ultra fine ferritecrystal grain structure is a well known technique. However, it wasconsidered to be difficult to form martensite structure overall in theordinary low carbon steel whose quenching property is not so good in theprocess of producing said ordinary low carbon steel sheet.

In order to produce high strength and high ductility low carbon steelhaving a tensile strength of 800 MPa or more, an uniform elongation of5% or more, and an elongation to failure of 20% or more from amartensite steel as a starting material, as the first step, inventors ofthe present invention have studied the relationship between martensitesteel as a starting material and the properties such as strength orductility of low carbon steel obtained by a subsequent treatment. And onsaid studying we have found out that said high strength and highductility low carbon steel having the expected strength, elongation andelongation to failure can be obtained from a steel whose martensitephase is 90% or more obtained by making the austenite crystalline grainscoarser, and then quenching into water followed by a cold rolling at atotal rolling reduction in thickness of 20% or more and less than 80%and by annealing, thus we have accomplished the object of the presentinvention.

Namely, the object of the present invention is accomplished by thecombination of said low strain processing and annealing and the specificsteel to be provided to said low strain processing and annealing.

DISCLOSURE OF THE INVENTION

The 1^(st) one of the present invention is a high strength and highductility low carbon steel sheet having a tensile strength of 800 MPa ormore and an uniform elongation of 5% or more, which is produced by amethod comprising, carrying out a low strain processing and annealing ona steel having a martensite phase in an amount of 90% or more obtainedby coarsening the size of an austenite crystal grain, which is existingin an ordinary low carbon steel or an ordinary low carbon steel addedwith boron in an amount of 0.01% or less being effective foraccelerating martensitic transformation, to 100 μm or more and thenquenching into water. Desirably, the 1^(st) one of the present inventionis the high strength and high ductility low carbon steel, wherein saidsteel possesses an ultra fine crystal grain ferrite structure having anaverage grain diameter of 1.0 μm or less formed by a low temperatureprocessing and annealing by carrying out a cold rolling at a totalrolling reduction of thickness of 20% or more and less than reduction ofthickness of 80%, and a low temperature annealing at the temperaturerange between 500° C. or more and less than 600° C.

The 2^(nd) one of the present invention is the method for producing ahigh strength and high ductility low carbon steel having a tensilestrength of 800 MPa or more and an uniform elongation of 5% or morecomprising, carrying out a low strain processing and annealing on asteel sheet having a martensite phase in an amount of 90% or moreobtained by coarsening the size of an austenite crystal grain, which isexisting in an ordinary low carbon steel or an ordinary low carbon steeladded with boron in an amount of 0.01% or less being effective foraccelerating martensitic transformation, to 100 μm or more and quenchinginto water, then carrying out a cold rolling at a total rollingreduction in thickness of 20% or more and less than 80%, and a lowtemperature annealing at the temperature range between 500° C. or moreand less than 600° C., to thereby form an ultra fine crystalline grainferrite structure having an average grain diameter of 1.0 μm or less.

BRIEF ILLUSTRATION OF THE DRAWINGS

FIG. 1 is the optical microscopic (OM) picture showing the structure ofthe longitudinal-vertical cross sectional view of the ordinary lowcarbon steel plate (JIS-SS400, 2 mm thickness) which is austenitized at1000° C. for 15 minutes, then quenching into water.

In the picture, RD indicates the rolling direction and ND indicatesnormal direction of the sheet.

FIG. 2 is the optical microscopic picture showing the structure of thelongitudinal-vertical cross sectional view of the cold rolled ordinarylow carbon steel (JIS-SS400) whose starting structure is a martensitestructure. (a) shows the case of 50% cold rolling and (b) shows the caseof 70% cold rolling.

FIG. 3 shows the nominal-stress-nominal-strain curves of the quenchedsteel of the ordinary low carbon steel (JIS-SS400) and cold rolled steelof various rolling reduction in thickness. a is a cold rolled steel at arolling reduction in thickness of 70%, b is a cold rolled steel at arolling reduction in thickness of 50%, c is a cold rolled steel at arolling reduction of thickness of 25%, d is a quenched steel ofmartensite structure, e is a steel as received of ferrite-pearlitestructure

FIG. 4 shows the nominal-stress-nominal-strain curves, a is a coldrolled steel at a rolling reduction in thickness of 50% of an ordinarylow carbon steel (JIS-SS400) whose starting structure is martensitestructure, and 30 minutes annealed steels of it (b; annealed at 400° C.,c; annealed at 500° C., d; annealed at 550° C., e; annealed at 600° C.).

FIG. 5 shows the relationship between annealing temperature andmechanical properties of a cold rolled and annealed steel at a rollingreduction in thickness of 50% of an ordinary low carbon steel(JIS-SS400) whose starting structure is martensite structure.

-●- is tensile strength (σ_(B)), -◯- is 0.2% proof stress (σ_(0.2)), -▴-is elongation of failure (e), -Δ- is uniform elongation (σ_(U)).

FIG. 6 is the transmission electron microscopic (TEM) picture showingthe structure of the longitudinal-vertical cross sectional view of acold rolled and annealed steel at a rolling reduction of 50% of anordinary low carbon steel (JIS-SS400) at various annealing temperatureswhose starting structure is martensite structure.

Annealed at the temperature of (a) 400° C., (b) 500° C., (c) 550° C.,(d) 600° C. for 30 minutes.

FIG. 7 is the graph showing the comparison of the relationship betweentensile strength and elongation to failure (strength-ductility balance)of rolled and annealed steel at a rolling reduction of 50% of anordinary low carbon steel (JIS-SS400) whose starting structure ismartensite structure at the various annealing temperatures for 30minutes (◯), and that of rolled and annealed steel at a rollingreduction with ARB of 97% of the steel whose starting structure isferrite-pearlite and annealed at various temperatures for 30 minutes(Δ).

FIG. 8 is a JIS 5 test piece for elongation test.

THE BEST EMBODIMENT TO CARRY OUT THE INVENTION

The present invention will be illustrated more in detail.

A. For the illustration of the present invention, the method for testand apparatuses for measurement are illustrated.

1. The shape of a test piece used for the tensile test is 1/5 size ofJIS 5 test piece (FIG. 8) (gage length 10 mm×gage width 5 mm).

2. The specimen for optical microscopic (Nikon Co., Ltd., Opti Photo100S) and TEM (Hitachi Co., Ltd., H-800) observation is prepared by awell-known method.

B. The important points of the present invention are illustrated withreference to the drawing.

The present invention will be illustrated along with following moreconcrete examples, however, following examples are mentioned only foreasy understanding of the present invention and not intending to limitthe scope of the present invention.

FIG. 1 is an optical microscopic picture showing the structure of thelongitudinal-vertical cross sectional view of a quenched steel which isobtained by using a hot rolled plate having 2 mm thickness of the rolledsteel material for a general construction use, namely, the steelmaterial containing miner constituents (JIS-SS400) such as

C; 0.13%, Si; 0.01%, Mn; 0.37%, P; 0.02%, S; 0.004%, sol. Al; 0.04%

as the receiving steel, and austenitization is carried out on said steelat 1000° C. for 15 minutes so as to make coarse the size of anauistenite crystal grain to 100-200 μm size, then water-quenched. Thispicture shows that the structure is the structure of coarse martensitestructure containing about 4% of proeutectoid ferrite.

FIG. 2 is an optical microscopic picture showing the structure of thelongitudinal-vertical cross sectional view of a cold rolled steelobtained by cold rolling of the receiving steel of FIG. 1 by multi passcold rolling by a total rolling reduction in thickness of 50% (a) and70% (b). The proeutectoid ferrite precipitated in prior austenite grainscan be observed in black contrast. In general, it is said that theworkability of martensite of carbon steel is not so good, however, fromFIG. 2 it is clearly understood that the low carbon steel martensite, atleast the low carbon steel martensite prepared according to the recipeof the present invention is possible to be cold rolled by reduction of70% or more.

FIG. 3 shows the nominal-stress—nominal-strain curves by tensile test ofquenched steel of FIG. 1 and cold rolled steel of FIG. 2. For thereference, the nominal-stress—nominal-strain curve e of a steel asreceived having ferrite-pearlite structure is shown by a dotted line.The tensile strength is improved from 410 MPa to 1100 MPa by quenching(d), further improved to 1340 MPa by cold rolling of 25% (c), to 1470MPa by cold rolling of 50% (b) and to 1640 MPa by cold rolling of 70%(a). While, elongation to failure is 10% around in the case of quenchedsteel and 6% around in the case of cold rolled steel. And the uniformelongation of the cold rolled steel is 1% or less.

FIG. 4 shows the nominal-stress-nominal-strain curves by tensile test ofa cold rolled steel obtained by rolling reduction of 50% of FIG. 3 andthe annealed steels of it treated at various temperatures for 30minuets. Although the strength is deteriorated by annealing, theductility recovers by annealing at 500° C. or more, and at thetemperature of 500° C.-550° C., the strength does not deteriorate somuch, while the elongation to failure and the uniform elongation areobviously increased. Accordingly, in annealed steel at 550° C. (d), theultra high strength-high ductility steel of 870 MPa tensile strength,710 Mpa 0.2% proof stress, 21% elongation to failure and 8% uniformelongation is obtained.

FIG. 5 shows the relationship between annealing temperature and tensilestrength (-●-), 0.2% proof stress (-◯-), elongation to failure (-▴-) anduniform elongation (-Δ-) of cold rolled steel by 50% and the annealedsteel thereof. When annealing temperature exceeds 525° C., elongation tofailure and uniform elongation are suddenly recovered, while tensilestrength is almost fixed at the temperature between the range from 500°C. to 550° C. This is the reason why the ultra high strength highductility steel is obtained.

FIG. 6 is the TEM picture showing the structure of thelongitudinal-vertical cross sectional view of a cold rolled and annealedsteel at a rolling reduction of 50%. The picture indicates that thestructure of 400° C. annealed steel (a) is a lamella structure similarto a heavily rolled steel. In the case of 500° C. annealed steel (b),ultra fine equiaxed grains of 100-300 nm are observed in broad range.Not shown in the drawing, it already becomes clear from the limitedrange of vision electron diffraction pattern that these ultra fineequiaxed grains are surrounded by large angle grain boundaries and arenot subgrains. The annealed steel at 550° C. has also similar ultra fineequiaxed grain structure, however, at the annealing temperature of 600°C., the coarser grain whose grain size is grown to several μm andspherically precipitated cementite are observed.

It is understood that the precipitation of cementite occurs at thehigher temperature than 500° C. so as to restrict the growth ofcrystalline grain, and consequently the ultra fine crystalline grainstructure of 100-300 nm is generated, further the work hardening abilitynecessary for uniform elongation is provided simultaneously.

As mentioned above, by the use of the low carbon steel of martensite asa starting material, and by low strain processing of 50% rollingreduction and annealing at 550° C., ultra fine ferrite crystalline grainstructure can be obtained, thus it becomes clear that it is possible toobtain a high strength and high ductility low carbon steel.

FIG. 7 shows strength-ductility balance of 50% cold rolled and annealedsteel of martensite which is a steel of the present invention (◯) andlarge strain processed steel (97% cold rolling steel) whose startingstructure is ferrite-pearlite structure of conventional art (Δ). Asmentioned above, when large strain processing is carried out usingferrite-pearlite structure as a starting structure, the structureobtained by annealing becomes mixed grain structure and desired highstrength and high ductility steel can not be obtained. On the contrary,in the case of cold rolled steel and annealed steel of martensite of thepresent invention, as clearly understood from FIG. 7, thestrength-ductility balance indicates experimental point which satisfiesthe conditions of 800 MPa or more tensile strength and 20% or moreelongation to failure is obtained.

INDUSTRIAL APPLICABILITY

As mentioned above, in an ordinary low carbon steel of 0.13C(JIS-SS400), an ultra fine ferrite crystalline grain structure of100-300 nm grain size can be obtained by annealing after 50% coldrolling using martensite structure of the present invention as astarting structure, and by annealing at 550° C. for 30 minutes, a steelwhich has excellent mechanical properties of 870 MPa tensile strength,21% elongation to failure and 8% uniform elongation is obtained. And itis obvious that the method for production of said steel providesexcellent effects, such as good economical advantage from the view pointof facility and a satisfaction of social requirement from the view pointof the environment and the circulation system of materials.

1-3. (canceled)
 4. A method for producing a high strength and highductility low carbon steel having a tensile strength of 800 MPa or moreand an uniform elongation of 5% or more and a elongation to failure of20% or more comprising, carrying out a low strain processing andannealing on a steel sheet product having a martensite phase in anamount of 90% or more obtained by coarsening the size of an austenitecrystal grain, which is existing in an ordinary low carbon steel or anordinary low carbon steel added with boron in an amount of 0.01% or lessbeing effective for accelerating martensitic transformation, to 100 μmor more and water-quenching.
 5. A method for producing a high strengthand high ductility low carbon steel of claim 4, wherein the annealing iscarried out at the temperature range between 500° C. or more and lessthan 600° C. after carrying out a cold rolling at a total rollingreduction in thickness of 20% or more and less than 80%.
 6. A method forproducing a high strength and high ductility low carbon steel of claim4, wherein an ultra fine crystalline grain ferrite structure having anaverage grain diameter of 1.0 μm or less is formed.
 7. A method forproducing a high strength and high ductility low carbon steel of claim4, wherein said steel sheet product having a martensite phase in anamount of 90% or more obtained by coarsening the size of an austenitecrystal grain steel, is obtained by using a hot rolled plate having 2 mmthickness, and is comprised of C 0.13%, Si; 0.01%, Mn; 0.37%, P; 0.02%,S; 0.004%, sol. Al; 0.04%, wherein an austenitization step is carriedout on said steel sheet product at 1000° C. for 15 minutes so as to makecoarse the size of an austenite crystal grain to 100-200 μm size, thenwater-quenched.